The basic concept of electrostatic spinning (or electrospinning) a polymer to form extremely small diameter fibers was first patented by Anton Formhals (U.S. Pat. No. 1,975,504). Electrostatically spun fibers and nonwoven webs formed therefrom exhibit very high surface areas and can be formed from a wide variety of polymers and composites. These materials have traditionally found use in filtration applications, but have begun to gain attention in other industries, including in nonwoven textile applications as barrier fabrics, wipes, medical and pharmaceutical uses, and the like. Due in part to the extremely high surface area of electrospun nonwoven webs, these materials also show promise in development of electrochemical materials such as electrochemical capacitors.
FIG. 1 illustrates a generic electrostatic spinning process including a rotating mandrel. The basic process consists of the use of a high voltage supplier 15 to apply an electrical field to a polymer melt or solution 30 held in a capillary tube 10, inducing a charge on the individual polymer molecules. Upon application of the electric field, a charge and/or dipolar orientation will be induced at the air-surface interface 60. The induction causes a force that opposes the surface tension. At critical field strength, the electrostatic forces will overcome surface tension forces, and a jet 40 of polymer material will be ejected from the capillary tube 10 toward a conductive, grounded surface such as a take-up reel 24. The jet 40 is elongated and accelerated by the external electric field as it leaves the capillary tube 10. As the jet 40 travels in air, some of the solvent can evaporate, leaving behind charged polymer fibers which can be collected on the take-up reel 24 driven by motor 26. As the fibers are collected, the individual and still wet fibers may adhere to one another, forming a nonwoven web 50 on the take-up reel 24.
Improvements to the basic process have been developed over the years. For instance, FIG. 2 illustrates a method of electrospinning fibers in an aligned orientation. According to this process, parallel conductive silicon plates on either side of an air gap (FIG. 2A) produce an electric field (FIG. 2B) that aligns the deposited fibers across the air gap (FIG. 2C) (see, e.g., Li, et al., Nanoletters, 2003, 3:8, 1167, which is incorporated herein by reference). This method has been used to collect two dimensional arrays of aligned and oriented fibers. Aligned nanofibers can show greater versatility as compared to random nonwovens formed by previous methods. For instance, oriented fibers are more favorable for tissue engineering applications due to the capability of directional guidance during tissue development. When considering textile or other materials applications, webs of oriented fibers present the possibility of developing anisotropic characteristics in the materials. Moreover, aligned fibers are more conducive to textile manufacture.
Dense three dimensional fiber arrays have also been formed by forming multiple layers of aligned fibers over one another in the air gap between the static conductive plates of FIG. 2A. Unfortunately, problems still exist with the aligned fiber arrays formed to date. For instance, methods to date provide limited fiber collection because as the charged, aligned fibers are piled upon one another, an increasing charge repels new fibers from being deposited on the formed mesh. Hence, arrays formed to date are of limited thickness. In addition, as fibers are collected one on top of another, the newly formed, still wet fibers adhere and bond to adjacent fibers above and below within the web. As a result, the as-fabricated arrays are very dense with little porosity. This prevents utilization of the arrays in applications that otherwise may be very well-suited to the extremely small diameter fibers. For instance, the dense arrays cannot be utilized in bioengineering applications as scaffolding material, as the dense mats can only allow cells to grow on the surface, and development of a three dimensional cellular construct throughout the depth of the array is not possible.
What are needed in the art are improved methods for forming three dimensional arrays of aligned nanofibers. In addition, what are needed in the art are three dimensional arrays of nanofibers that can be formed to any desired depth with an open, loose structure.